[MPlayer-dev-eng] [PATCH] Remove leading/trailing margin for VCD

[Zuxy Meng]

Hi,

Playing around with VCD, I found that the MPEG packs don't start
precisely at the beginning of a track, and don't end precisely at the
end of a track either: they are wrapped by all zero sectors. I don't
have a copy of the White Book so I can't tell if it's mandated by the
standard or just an implementation issue. I guess the rationale behind
these margins is that old VCD players have problems in accurate
seeking. Nevertheless, the leading margin makes lavf demuxer reject it
as a legitimate MPEG PS stream.

Two notes about the patch:
1. Experience shows that reading sector by sector in the backward
direction is extremely slow so I commented such code out. As a result
trailing margin isn't stripped and fortunately it doesn't cause much
trouble in demuxing too.
2. Each MPEG pack (as well as a non-null VCD sector) begins with
0x000001ba and I picked the last byte (0xba) as the probe.
-- 
Zuxy
Beauty is truth,
While truth is beauty.
PGP KeyID: E8555ED6
-------------- next part --------------
Index: stream/stream_vcd.c
===================================================================
--- stream/stream_vcd.c	?????? 22543??
+++ stream/stream_vcd.c	????????????
@@ -79,7 +79,7 @@
 
 static int open_s(stream_t *stream,int mode, void* opts, int* file_format) {
   struct stream_priv_s* p = (struct stream_priv_s*)opts;
-  int ret,ret2,f;
+  int ret,ret2,f,sect,tmp;
   mp_vcd_priv_t* vcd;
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
   int bsize = VCD_SECTOR_SIZE;
@@ -135,6 +135,19 @@
     m_struct_free(&stream_opts,opts);
     return STREAM_ERROR;
   }
+#if 0 // search backward incurrs significant performance degradation!
+  /* search backward up to 3 seconds to skip trailing margin */
+  sect = ret2 / VCD_SECTOR_DATA;
+  for (tmp = sect - 1; tmp > sect - 3 * 75; tmp--) {
+    char mem[VCD_SECTOR_DATA];
+    vcd_set_msf(vcd, tmp);
+    /* If the sector contains an MPEG pack, mem[3] == 0xBA != 0 */
+    if (vcd_read(vcd, mem) != VCD_SECTOR_DATA || mem[3] != 0)
+      break;
+  }
+  mp_msg(MSGT_OPEN, MSGL_DBG2, "%d trailing sectors skipped\n", sect - tmp);
+  ret2 = (tmp + 1) * VCD_SECTOR_DATA - 1;
+#endif
   ret=vcd_seek_to_track(vcd,p->track);
   if(ret<0){
     mp_msg(MSGT_OPEN,MSGL_ERR,MSGTR_ErrTrackSelect " (seek)\n");
@@ -143,6 +156,17 @@
     m_struct_free(&stream_opts,opts);
     return STREAM_ERROR;
   }
+  /* search forward up to 3 seconds to skip leading margin */
+  sect = ret / VCD_SECTOR_DATA;
+  for (tmp = sect; tmp < sect + 3 * 75; tmp++) {
+    char mem[VCD_SECTOR_DATA];
+    if (vcd_read(vcd, mem) != VCD_SECTOR_DATA || mem[3] != 0)
+      break;
+  }
+  mp_msg(MSGT_OPEN, MSGL_DBG2, "%d leading sectors skipped\n", tmp - sect);
+  vcd_set_msf(vcd, tmp);
+  ret = tmp * VCD_SECTOR_DATA;
+
   mp_msg(MSGT_OPEN,MSGL_V,"VCD start byte position: 0x%X  end: 0x%X\n",ret,ret2);
 
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)